Detective Acting Sergeant Ryan said the fire was believed to have started in at least three places just to the west of Scoullers Road, off the Cobden-Stoneyford Road near Stoneyford.

"It is unknown how the fires started. There's nothing to confirm it was deliberately lit but the cause of the fire is definitely suspicious," he said.

"We looked at one site late last night and the CFA investigator described it as highly likely as one of the seats of the fire," he said.

"While the cause of the fire is suspicious, at this stage we do not know how it started. Whether it was deliberate or some other unknown means we are unsure as yet."

Detective Acting Sergeant Ryan said the fire started about 2.40pm on Wednesday and he called for any witnesses who saw anything unusual in that area, or a vehicle, to contact the Warrnambool police station CIU or Crime Stoppers.

On Wednesday night more than 30 CFA units and a number of aircraft attacked the fire that was swept along by strong south east winds.

The fire has blackened about 450 hectares of farmland near the Cobden-Stoneyford Road and is burning in stony terrain.

At one stage it threatened three houses near the corner of Scoullers Road and the Cobden-Stoneyford Road.